In a semiconductor pressure sensor disclosed in U.S. Pat. No. 6,635,910, a heavily-doped region serving as a gauge section is formed to a surface portion of a semiconductor substrate by an ion-implantation process, a thermal diffusion process, and the like. The semiconductor substrate of the semiconductor pressure sensor is attached to a thinned portion of an object to be detected. When receiving pressure, the thinned portion of the object is deformed according to an amount of the received pressure. The semiconductor substrate (i.e., the gauge section) attached to the thinned portion is deformed accordingly. An electrical resistance of the gauge section changes according to a degree of the deformation of the gauge section. Therefore, the pressure applied to the object can be detected based on the electrical resistance of the gauge section. A thickness of the surface portion of the semiconductor substrate, where the gauge section is formed, is approximately 2 micrometer (μm), and a total thickness of the semiconductor substrate is between 10 μm and 14 μm.
It is difficult to adjust a depth to which ions are implanted to form the heavily doped region. In such a conventional pressure sensor, therefore, after the ion-implantation process is finished, the thermal diffusion process is performed to achieve uniform impurity concentration of the heavily doped region.
However, it is difficult to control the diffusion in the semiconductor substrate. Therefore, even when the thermal diffusion process is performed, variations in impurity profile may occur. The variations in the impurity profile result in variations in the electrical resistance of the gauge section. The variations in the electrical resistance of the gauge section cause a reduction in detection accuracy. The same problem can arise for other semiconductor sensors such as a semiconductor temperature sensor, which detects a volume change due to a temperature change of an object as an electrical resistance change of a gauge section.
In a pressure sensing apparatus disclosed in U.S. Pat. No. 4,840,067 corresponding to JP-A-H7-11461, such a semiconductor pressure sensor is attached to a diaphragm integrally formed with a metallic stem through a low-melting glass. To reduce thermal stress, the metallic stem (i.e., diaphragm) is required to be made of a material having a thermal expansion coefficient close to a thermal expansion coefficient of a semiconductor substrate (i.e., silicon) of the semiconductor pressure sensor. Therefore, the metallic stem is made of a special material such as kovar, which is more costly and has less workability than a general material such as a stainless steel (SUS). Since a housing for accommodating the metal stem is made of such a general material, the stem cannot be fixed to the housing by welding. Further, the low-melting glass is melt during a bonding process in which the semiconductor pressure sensor is attached to the diaphragm of the stem. Therefore, the low-melting glass may not be accurately positioned with respect to the diaphragm. As a result, the semiconductor pressure sensor may be displaced from a correct location on the diaphragm. The displacement of the semiconductor pressure sensor causes a reduction in detection accuracy.